The present invention relates generally to high frequency circuits and more particularly to output drivers in high frequency circuits.
Digital communication among integrated circuits operating in high data rates requires transmission lines, which are connected to the circuits' output drivers. The characteristic impedances of the transmission lines should be matched to impedances of the output drivers.
FIG. 1 shows a typical CMOS output driver connected to an output transmission line with a characteristic impedance Zo. In operation, when an input voltage Vi is at a low logic level, the pull-up section of the CMOS output driver (the PMOS transistor) is at a low impedance state. The impedance of the low impedance state is substantially equal to Zo to serially terminate the impedance of the output transmission line. With the PMOS transistor at the low impedance state, Vcc is coupled to the output transmission line. By voltage division, half of Vcc travels down the output transmission line from point X. The other end of the output transmission line, point Y, is usually a gate of another circuit, which typically has a very high impedance. When half of Vcc reaches point Y, it will be reflected back through the output transmission line. Due to the reflected signal directly adding to the incident signal, the level of voltage at point Y becomes approximately Vcc, the high logic level. This Vcc signal propagates back towards point X. Once it reaches point X, this signal turns the PMOS off, and the circuit in FIG. 1 is in its stable state. Similarly, if Vi is at the high logic level, point Y, in its stable state, would be at the low logic level.
If there are a plurality of CMOS output drivers connected to an output transmission line, each driver should have a switch. There is a need to effectively connect the plurality of output drivers with switches to drive the output transmission line.
It will be apparent from the foregoing that there is still a need for a high frequency circuit that effectively combines output drivers with switches to drive an output transmission line.